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A blockchain is about consensus on the starting point, the genesis block, and all the blocks after. There is virtually nothing that prevents anyone from creating a new genesis block and blockchain, but your blockchain's value is based on the value that people place on it.

Resetting testnet usually means the Ethereum core developers changing the behavior of say geth --testnet to use a new genesis block. Most people won't care if this was done: testnet ETH have no value because they can be obtained for free using faucets that have been set up so that people can freely test Ethereum and DApps.

If someone tried to do the same thing for mainnet, people would likely not run that client software. Those who do want to run such a client, will be on another network, and they can interact with themselves but unlikely to get followers: for example, if you have some ETH now and zero ETH on another network, you're not likely to switch.

That means that only the consensus that this is a worthless testnet prevents the testnet from becoming a mainnet at some point. If I would build a dao that proves to be profitable suddenly morden could be valuable.
– Roland KoflerJun 1 '16 at 4:41

this argument is flaky: "testnet ETH have no value because they can be obtained for free using faucets". Faucets have nothing to do with value because the source of ETH in the faucets is mining reward and not some costless procedure. In fact, faucets for real bitcoins existed for a long time, the idea was to incentivise a broader adoption.
– Roland KoflerJun 1 '16 at 6:15

so theoretically Elon Musk and Andreas Antonoupulous could collude and say: we build a business on testnet now. And suddenly testnet would be unresetable and gain value.
– Roland KoflerJun 1 '16 at 6:18